Vehicles are equipped with air conditioners for cooling and dehumidification in the summer.
Air conditioners include compressors, condensers, expansion valves, and evaporators, circulate refrigerant through them, and supply cold air, which is generated by absorbing ambient heat when the refrigerant evaporates in the evaporators, to the interior.
The temperature of air discharged to the interior is preferably uniform regardless of the position of each vent. However, if the temperature in an evaporator is not uniformly distributed, the temperature distribution of air passing through a heat exchanger is not uniform and hence the temperature of discharged air may vary depending on the vents.
Accordingly, it is necessary to uniformize a temperature distribution throughout the area of the evaporator, i.e. the heat exchanger through which air passes.
In recent years, a plurality of rows of heat exchangers is used to help in uniformizing the temperature distribution of air therethrough. Typically, first and second row heat exchangers are installed to overlap each other, and these two heat exchangers form a single system having a single inlet and outlet.
FIG. 1 is a diagram illustrating a plurality of rows of heat exchangers according to the related art, and illustrates that first and second row heat exchangers arranged in forward and backward directions are separated on one plane.
As illustrated in the drawing, each of a first row heat exchanger 10 and a second row heat exchanger 20 includes an upper header tank, a lower header tank, and a plurality of tubes connecting them. The first and second row upper and lower header tanks are partitioned into a first row upper space 11, a second row upper space 21, a first row lower space 12, and a second row lower space 22 by a partition wall that traverses the center portion therein.
Baffles 31 and 32 are installed at predetermined positions in the spaces to block the flow of a refrigerant, so that a plurality of passes having the flow of a refrigerant is formed in the respective spaces. By way of example, a heat exchanger is illustrated to have a total of six flow paths consisting of three passes in a first row and three passes in a second row. A refrigerant inlet 11a is formed at one side of the first row upper space 11, a refrigerant outlet 21a is formed at one side of the second row upper space 21, and a communication hole 40 for connecting the first and second row lower spaces 12 and 22 therethrough is formed at one side of the partition wall of the lower header tank.
Accordingly, the refrigerant introduced into the refrigerant inlet 11a passes through {circle around (1)}, {circle around (2)}, and {circle around (3)} passes of the first row heat exchanger 10, flows to the second row heat exchanger 20 through the communication hole 40, passes through {circle around (4)}, {circle around (5)}, and {circle around (6)} passes, and is then discharged to the refrigerant outlet 21a. 
However, since the conventional heat exchanger has a series flow structure in which a refrigerant flows to the second row heat exchanger 20 via the first row heat exchanger 10, there are regions ({circle around (1)} and {circle around (6)} passes) where a severe variation in temperature occurs among the overlapped passes ({circle around (1)} and {circle around (6)}, {circle around (2)} and {circle around (5)}, and {circle around (3)} and {circle around (4)}) in the state in which the heat exchanger is installed in the vehicle.
For this reason, the temperature distribution in the heat exchanger may not be uniform, and the temperature distribution of air passing through the heat exchanger may not be uniform.
Korean Patent Application Publication No. 10-1998-0050607 discloses a heat exchanger having an overlapped structure in first and second rows.